1. Field of the Invention
This invention relates to an image forming apparatus having a thermal fixing step in an image forming step done at such as, e.g., printers, facsimile machines, and photocopiers.
2. Background of Related Art
In conventional electrophotographic type image forming apparatuses such as, e.g., printers, facsimile machines, and photocopiers, developments are made by attaching toner as a developer to electrostatic latent images formed on a surface of a photosensitive drum as an image carrier. The developer images produced by the development step are transferred onto a recording medium and conveyed to a fixing unit.
The fixing unit generally includes a fixing roller having a heater as a thermal source, and a pressure roller nipping recording media together with the fixing roller with pressure. The recording media transferred with developer images are given with heat and pressure when passed through a nipping portion formed between the fixing roller and the pressure roller, thereby fixing the developer images on the recording media.
Some image forming apparatuses thus formed detect surface temperature of the pressure roller at a start timing of warming up of the apparatus, do warming up based on the detected results, and start printing operation (see, e.g. Japanese Patent Application Publication No. 2011-76019 (A1)).
With such a conventional apparatus, however, there raises a problem that warming up based only on the detected results tends to cause fixing failures at the printing initial stage or paper deformations or paper curling, even where warming up is made based on the detected results of the surface temperature of the pressure roller.
It is therefore an object of the invention to provide an image forming apparatus capable of suppressing occurrence of fixing failures and paper deformations even without any detection result of pressure roller's surface temperature.